Many different types of transparent image recording sheets, or "transparencies" as they are called in the industry, are known in the art. Transparencies can be made by different printing and imaging methods, such as thermal transfer printing and ink jet printing as well as color photocopying and plain paper copying, e.g., electrography and xerography. All of these transparencies are suitable for use with overhead projectors.
In copying procedures, the formation and development of xerographic images uses a toner composition containing resin particles. Toners are generally applied to a latent image generated on a photoconductive member. The image is then transferred to a suitable substrate, such as a transparent image receptor, and affixed thereon, by the application of heat, pressure, or a combination thereof. These transparent image receptors generally include a polymeric substrate, such as polyethylene terephthalate, and have an image-receiving layer coated thereon for better toner adhesion.
In thermal transfer imaging or printing, an image is formed on a receptor sheet, e.g., transparency, when a donor sheet or ribbon, having a colorant (e.g., dye or pigment) layer thereon, is brought into intimate contact with the receptor sheet and heated with a localized heat source, such as a laser or a thermal print head. The heat source directly contacts the backside of the donor sheet. A thermal print head contains small, electrically heated elements that can be selectively heated, thereby transferring colorant, either alone or in association with carrier materials, from the donor sheet to the receptor sheet in an image-wise manner. This imaging process can involve either mass transfer of colorant in a binder or state-altered transformation of a dye, as by melt transfer, diffusion, or sublimation of the colorant, for example. In a mass transfer process, the colorant, e.g., dye or pigment, is dispersed within a binder, as in a toner, and both the colorant and its binder are transferred from a donor sheet to a receptor sheet. In a dye transfer process, the colorant (the "dye" present on the donor with or without a binder) is transferred without binder by melting, melt-vaporization, propulsive ablation, sublimation, or vaporization, for example, to a receptor sheet where the colorant adheres to, or diffuses into, the image-receiving layer.
Such thermal transfer systems generally require the use of receptor sheets with certain specific requirements. For example, the receptor sheet should be designed to effectively receive an image from a donor sheet and to hold the image and yield a desired print with generally high optical image density, brightness, and stability. Preferably, it should also be generally scratch resistant and have little or no tack or static buildup, particularly if the receptor sheet is a transparency. In a typical receptor sheet, an image-receiving layer is coated on a substrate, preferably a flexible substrate that is formed from a film-forming material, such as paper, polymeric film, and the like, although for transparencies, the substrate is a transparent polymeric film. The image-receiving layer typically includes a polymeric resin, e.g., a thermoplastic film-forming resin, that is compatible with colorants and adheres well to the substrate, either directly or through the use of an intermediate adhesive layer.
Although there are a host of receptor sheets available for use in thermal mass printing and electrographic or xerographic copying, there remains a need for new receptor sheets bearing image-receiving layers that enable the formation of generally high quality black and white or color images, with low tack, good feedability, good scratch resistance, low haze, and good adhesion to the substrate. Furthermore, there is a need for an image-receiving layer that can be coated out of an aqueous composition, i.e., a latex, as most image-receiving layers are coated out of organic solvent formulations. Organic solvent formulations are undesirable at least because such formulations use expensive, toxic, volatile, and often flammable organic solvents, which typically must be captured upon drying to prevent air pollution and health and safety problems.
Water-based coating compositions for various nonimaging applications are known, e.g., wood and rubber coatings; however, there are few water-based coating compositions known and used in thermal transfer systems. Many water-based coating compositions include "soft" latex polymers, i.e., polymers having a low glass transition temperature (i.e., a "Tg" of about 30.degree. C. or lower). Such compositions, however, are generally too tacky for use in the image-receiving layer on receptor sheets, particularly transparencies. That is, polymeric substrates coated with an image-receiving layer containing a soft polymer typically stick together and thus cannot be fed easily into a printer.
A typical solution to this problem is to blend a high Tg latex, i.e., a polymer having a Tg of about 45.degree. C. or higher, with the low Tg latex. Such a blending of latexes of low and high Tg polymers, or designing latex polymers that include segments of widely differing glass transition temperatures in different proportions, provides coatings of varying degrees of hardness and softness. This approach can result in certain desirable properties, e.g., low tack, high hardness, etc. For example, carboxylate latexes of butadiene/itaconic acid/methacrylic acid/styrene copolymers (Tg of -10.degree. C. to -16.degree. C.) blended with a latex of ethyl acrylate acrylamide and methyl methacrylate in equal amounts (Tg&gt;45.degree. C.) produces nontacky coatings. This method of mixing low and high Tg polymers can also result in undesirable properties, however, particularly if used in coating compositions for imaging applications. For example, if the low and high Tg polymers are incompatible, a nonuniform microstructure and haze can result, which can lead to poor quality images and discreet tacky regions on the surface of the coating.
Other methods of decreasing the tackiness of water-based coating compositions include the use of additives such as N-dodecylsulfosuccinamate, silicon dioxide, adipoyl dihydrazide, polyvinyl alcohol, and a blend of sodium linoleate, CaCO.sub.3, and silica. Basic materials such as adipoyl dihydrazide, however, are limited to use in cationic latexes, which are generally few in number. Molecules containing hydrocarbon chains such as N-dodecyl sulfosuccinamate can cause compatibility problems with acrylic materials. The use of calcium carbonate materials may not be suitable for sufficiently transparent coatings.
Thus, what is needed is an image-receiving layer that can be coated out of an aqueous coating composition to produce a generally nonblocking, i.e., low tack or nontacky, receptor sheet, particularly a transparency, imageable with generally good image quality and capable of withstanding the printing process with little or no scratching. The resultant image-receiving layer should exhibit generally good adhesion to the surface of a substrate, to the donor surface during imaging, and also to colorants, e.g., toners, transferred during the imaging process.